Various coatings are applied to metal substrates. It is highly desirable that the metal substrate be effectively cleaned of contaminants prior to the application of the coating so that the useful coating life may be prolonged. Contaminants include liquid halogens, sulfur compounds, and occasionally nitrogen compounds. Such surface contaminants include water soluble salts, such as chlorides, sulfates and nitrates. On steel substrates such salts are iron salts (ferrous and ferric salts.)
Various coatings are also applied to concrete substrates. Concrete is of course different from metals such as steel in that it is not chemically reactive with water soluble salts such as sodium chloride.
The presence of water soluble salts on substrates has long been recognized as a major factor in reducing coating life. The detrimental effect of these contaminants on coating performance has been discussed in coatings related literature for almost 30 years. Water soluble salts on a substrate initiate coating disbondment (and substrate corrosion) through an osmotic blistering process which is described below.
In the event of a media blasted steel substrate, ferrous chloride is formed whenever steel or iron and soluble chloride in moisture are in contact. This reaction, in itself, is a strong corrodant of steel surfaces. Upon exposure to air, ferrous chloride oxidizes to ferric chloride, a hygroscopic salt with a natural affinity for moisture in the air. Trace amounts of either ferric or ferrous chloride remaining on the substrate accumulate moisture from the air resulting in the formation of a concentrated iron chloride solution on the surface of the steel substrate. Iron ions, chloride ions and water comprise an electrolytic solution that drives an electrochemical corrosion reaction. Coatings applied over such a substrate fail in a short period of time due to the concentrated iron chloride solution on the substrate drawing water through the coating by osmosis and creating a blistering or disbondment of the coating. Rates of coating failure due to osmotic blistering are dependent on the thickness and porosity of the coating.
Concrete is a cast material that is porous by nature. The porosity of concrete may provide water and air pockets extending from the surface into the material to a depth of one inch. The amount of porosity varies with the method of casting of the cement and the type of finish applied. Hard troweling of the surface minimizes porosity.
Contamination of substrates from soluble salts has been identified as the source of coating failure and has been thoroughly documented. Practical cost effective solutions to the problem have eluded routineers in the coating science field. Complicating the search for cost effective solutions is the lack of standards defining acceptable levels of soluble salt contaminations or concentrations on substrates. The level of cleanliness required varies significantly with the service environment and the characteristics of the coating selected. However, independent of these variables, "the cleaner the substrate, the greater the resistance to coating disbondment".
Until recently, blast cleaning specifications have not addressed removal of non-visible surface contaminants. Conventional grit blasting techniques were not designed to remove ionic contamination. Dry abrasive blasting can not efficiently remove localized sources of corrosion initiation sites (commonly referred to as corrosion cells) because an operator may not be able to see such contaminants and direct a dry grit blast against such corrosion initiation sites. Efforts to develop methods for removal of these non-visible contaminants from substrates, both metal and concrete, have been generally unsuccessful although several techniques have been tried with partial success, such as, for example, (1) dry blasting followed by water rinsing (several cycles), (2) hard grit wet abrasive blasting, (3) high pressure washing, and (4) acid washing followed by water rinsing.
The coating performance of concrete substrates is affected primarily by two problems. One problem involves the formation of a thin layer of non-reactive materials on the surface of cured new concrete as a residue. The residue forms a weak powdering material with little adhesive strength and therefore is not acceptable for the subsequent application of a coating material over the surface of the concrete. The other problem is that uncleaned concrete of any age contains water soluble salts in the voids. These salts create the same hygroscopic condition that salt contaminants in steel create as a microscopic layer of water is always present on the substrate surface regardless of temperature and humidity conditions, due to the hydroscopic nature of the salt contaminants. Coatings applied over salt contaminated surfaces fail in a short period of time due to poor adhesion caused by osmotic blistering.
Also, particularly when horizontal concrete surfaces are etched with acid, such as hydrochloric or muriatic acid, the reaction of the acid with the cement creates soluble salts which are present in the pores of the concrete. The removal of such soluble salts heretofore has been attempted by the use of a stiff bristle broom and copious amounts of rinse water which in many instances have been ineffective to remove the salts.